To prevent the generation of an alarm even without masking when not connecting any additional terminal by looping back the corresponding output signal of an auxiliary system interface(I/F) means for main terminal to the input port of the auxiliary system I/F means for main terminal when not connecting any additional terminal.

When an additional terminal 3-1 is disconnected or is not connected from the beginning, it is detected by a control part and a selector 11-1 respectively switches relays 803 and 804 to the side of terminal T and switches relays 703 and 704 to the side of terminal E. Thus, a part time signal (single dotted line) sent from an auxiliary system I/F part 10-1 for main terminal is returned (looped back) to the input port of the auxiliary system I/F part 10-1 for main terminal while keeping the transmission line of a main signal. Thus, the presence of input is continuously detected at the auxiliary system I/F part 10-1 for main terminal and the generation of the alarm caused by input interruption can be prevented.